Abstract (English)

Within the scope of this thesis, different approaches towards photochemical and chemical water oxidations have been investigated. Metal-complex-functionalized vesicles catalyzed visible light-driven water oxidation. A replacement of the single molecule photosensitizers by quantum dots was investigated and an photochemical recycling of Ce(IV) was studied.
Chapter 1 reviews the development of ...

Abstract (English)

Within the scope of this thesis, different approaches towards photochemical and chemical water oxidations have been investigated. Metal-complex-functionalized vesicles catalyzed visible light-driven water oxidation. A replacement of the single molecule photosensitizers by quantum dots was investigated and an photochemical recycling of Ce(IV) was studied.Chapter 1 reviews the development of chemical and photochemical systems for artificial photosynthesis at dynamic self-assembled interfaces. The chapter highlights the most important reports on water oxidation, photocatalytic hydrogen production, overall water splitting systems and photocatalytic CO2 reduction in vesicular and micellar systems, respectively. The role of the interface such as charge separation, back electron transfer suppression, solubilization or protection of sensitive intermediates is discussed for every mentioned system and a short outlook is provided.Chapter 2 deals with the development of functionalized phospholipid bilayer vesicles for visible light-driven water oxidation. Molecular water oxidation catalysts and photosensitizers were modified with alkyl chains and co-embedded into phospholipid membranes to prepare functionalized small unilamellar vesicles. These aggregates photocatalytically produced molecular oxygen when irradiated with blue light from LEDs in phosphate buffer. The two dimensional assembly of photosensitizers and catalysts at the vesicle-water interface allowed photocatalytic water oxidation at very low overall catalyst concentrations of 500 nM, which are inoperable in homogeneous solutions. Functionalized, rigid gel phase membranes obtained the highest TONs. This indicates that phase separation enhances the photocatalytic activity of the assembly by clustering and limited dynamics of the embedded compounds. The concept of membrane co-embedding can be applied to various combinations, ratios and concentrations of photosensitizers and water oxidizing catalysts, providing a new approach to artificial photosynthesis.Chapter 3 describes the attempt to use colloidal quantum dots as photosensitizers in photochemical water oxidation. Differently-sized CdSe quantum dots with several ligand environments were synthesized and characterized. Vesicular and homogeneous systems for photochemical water oxidation with quantum dots and homogeneous water oxidation catalysts have been prepared. Different combinations and ratios of quantum dots and catalysts were investigated towards oxygen evolution. A fluorescence quenching study revealed a charge transfer from quantum dots to methyl viologen and to a water oxidation catalyst, respectively. Regardless of the combinations and ratios of different quantum dots and water oxidation catalysts, no working photocatalytic water oxidation system was achieved.Chapter 4 deals with the photocatalytic regeneration of the chemical oxidant Ce(IV), a commonly used agent for the chemical oxidation of water. Typical water oxidation catalysts reach higher catalytic activities under chemical water oxidation conditions. Therefore, the photochemical recycling of Ce(IV) could enhance visible light-driven water oxidation reaction. Different approaches to detect the generation of Ce(IV) by UV/Vis spectroscopy under photochemical water oxidation conditions were examined. Unfortunately, no system could fulfill the requirements due to unwanted side-reactions during the irradiation. The photochemical regeneration of ceric ammonium nitrate (CAN) with the photocatalysts triphenyl pyrylium (TPP) or 9-mesityl-10-methylacridinium (Acr+-Mes) was investigated. The system utilizing TPP proved to be not suitable due to the interaction of CAN and the light of 400 nm wavelength. The Acr+-Mes containing solutions also decreased the turnover of the water oxidation catalyst. In conclusion, the photochemical recycling of Ce(IV) could not be realized.